Standards set by the National Fire Protection Association (NFPA) as detailed in the National Electric Code, Article 517-104 require that each power circuit within an anesthetizing location, such as in a hospital operating room, shall be ungrounded and isolated from any distribution system supplying other, non-anesthetizing locations. Additionally, there is a requirement that a monitor must be used with the ungrounded system to provide a continuous indication of possible leakage or fault currents from any of the isolated conductors to ground. This requirement is for the safety of the patient. The normal high impedance of the human body can be bypassed during certain medical procedures such as when electrodes or probes are used to monitor heart activity, for instance. Under these conditions, alternating current flowing through the body could produce extreme shock or even death.
The ideal isolated conductor in an ungrounded system has infinite impedance to ground and no current flow would result if a short circuit or very low impedance was placed between the conductor and ground. In the real world, however, there are no perfect insulators. All isolated conductors experience some capacitive or resistive leakage current to ground. Insulation deteriorates with age and use. Capacitive leakage are inherently present in all systems. Both types of leakages increase by the number of devices connected to the system and the length of the conductors themselves. These leakages provide a current path to ground and if a grounded low impedance is connected to the isolated conductor, as might be the case with a heart monitor, the current has a return path due to the leakages. Total hazard current is defined as the total current that would flow through a low impedance conductor connected between ground and the isolated conductor. Total hazard current is a combination of fault hazard current, that current that results from all user devices, except for a LIM, connected to the isolated system, and monitor hazard current, that current that results only from a LIM connected to the isolated system. The NFPA standards require a visual and audible warning if the total hazard current exceeds a predetermined limit and also limits the amount of current attributed to the monitor.
Several types of LIMs have been available for some time. These include static ground detector and dynamic LIMs. These types, although somewhat effective, did not offer continuous monitoring, were somewhat difficult to use, and also added a large amount of hazard current to the total hazard current. One type of dynamic LIM that overcame some of these problems is disclosed in commonly assigned U.S. Pat. No. 3,976,987. A DC reference voltage, proportional to the maximum voltage to ground from either conductor of a two wire ungrounded system is determined and used as a reference voltage for the system. A capacitive component and a resistive component are generated from the reference voltage and are applied across the system leakage impedances in parallel. Circuitry within the LIM separates the impedance voltage response from the line voltage response to produce a difference signal. This signal is used to derive a combined resistive and capacitive component signal representing the maximum hazard current of the system. However, this type of monitor requires periodic manual calibration which requires removal from the system for calibration at a test bench. The monitor is frequency and voltage dependant, thereby requiring different versions to accommodate different voltages and line frequencies. Furthermore, the monitor and others similar to it have limited self-test capabilities. These self checks could determine if the monitor itself is operating properly. Some multiphase systems require more than one monitor on the same circuit. Prior art LIMs were not capable of being coupled to a circuit at the same time due to LIM to LIM interference. If the ground connection to the LIM itself is defective, measurement of the hazard current will be unreliable. It would be desirable to provide a line isolation monitor that overcomes these disadvantages and includes other features that accurately computes the hazard current of an ungrounded power system.